1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image recorded on a recording material, and more particularly to an image heating apparatus adapted for use as a heat fixation apparatus to be mounted in a copying apparatus or a printer employing an electrophotographic recording technology or an electrostatic recording technology.
2. Related Background Art
A heat fixation apparatus employed in recent copying apparatus or printer is required to secure a satisfactory fixing property with an even less electric power consumption.
For attaining such performance, there is, for example, known a fixing apparatus of an external heating method as disclosed in Japanese Patent Application Laid-open No. H10-133505. Such fixing apparatus is provided with a fixing roller, a pressure roller for forming a recording material conveying nip portion together with the fixing roller, and a heating roller for heating an external periphery of the fixing roller, wherein the fixing roller is equipped with an elastic layer for obtaining a nip width for securing the fixing property and the fixing roller is heated from a surface thereof in order to promptly raise the surface of the fixing roller to a fixing temperature.
However, in the above-described method of heating the surface of the fixing roller from the exterior, a smear on an external heating roller is a concern.
In the recording material conveying nip of the fixing apparatus, there generally stands a relationship “adhesive force between toner and recording material”>“adhesive force between the surface of fixing roller or pressure roller and toner.” As a result, the toner does not stick to the surface of the fixing roller but to the recording material.
On the other hand, in case of a detection of an abnormal situation such as a sheet jam, an image forming apparatus suspends its operation and terminates the operation of a heater etc. for security. In such situation, if a sheet bearing an unfixed toner is present in the fixing nip portion, the toner may be deposited (offset) onto the surface of the fixing roller by a temperature decrease of the roller. Also a toner deposition may occur on the fixing roller at a jam resolving procedure by the user.
In a conventional fixing apparatus without an external heating member, even in case the surface of the fixing roller or the pressure roller is smeared with an offset toner, for example, by a sheet jam, the aforementioned relationship of forces is restored when the apparatus returns to a normal state, whereby the offset toner on the fixing roller or the pressure roller is fixed onto a first passed sheet and can therefore be removed from the roller.
However, in a fixing apparatus provided with an external heating member, the offset toner resulting, for example, in a sheet jam may also stick to an external heating roller. In such case, in a first passed sheet after jam processing, the toner on the external heating roller, not coming in direct contact with the recording material, cannot be removed completely. The toner remaining on the external heating roller returns onto the fixing roller irregularly, thus smearing an image on the recording material.
In order to avoid such drawbacks, it is also proposed to provide a cleaning member in contact with the fixing roller or the external heating roller thereby eliminating the offset toner, but the apparatus becomes larger and more complex and the cleaning member has to be replaced periodically by the user.